Color, digital image display devices are well known and are based upon a variety of technologies such as cathode ray tubes, liquid crystal and solid-state light emitters, such as Organic Light Emitting Diodes (OLEDs). In a common OLED display device, each display element or pixel, is composed of red, green, and blue colored OLEDs. By combining the illumination from each of these three OLEDs in an additive color system, a wide variety of colors can be achieved.
OLEDs may be used to generate color directly using organic materials that are doped to emit energy in desired portions of the electromagnetic spectrum. However, the known red and blue emissive materials do not have particularly high luminance efficiencies. However, materials with higher luminance efficiencies are known in the art. While power efficiency is always desirable, it is particularly desirable in portable applications because an inefficient display limits the time the device can be used before the power source is recharged. Portable applications may also require the display to be used in locations with high ambient illumination, requiring the display to provide imagery with a high luminance level to be useful, further increasing the power required to present adequate imagery.
When designing a display device, it is important to understand the colors that are perceived by a human observer and the human eye's sensitivity to these colors. FIG. 1 shows a 1931 CIE standard photopic sensitivity curve 2. This curve relates the relative efficiency of the human eye to convert electromagnetic energy to perceived brightness as a function of wavelength within the visible spectrum. Electromagnetic energy that is weighted by this curve is commonly referred to as luminance, an entity that correlates with perceived brightness under a broad range of viewing conditions.
Traditionally, display devices have been constructed from a triad of red, green, and blue light emitting elements. The peak wavelengths of these light emitting elements will typically be in the short wavelength portion of the visible spectrum (e.g., at or near point 4) for blue, the middle wavelength portion of the visible spectrum (e.g., at or near point 6) for green, and the long wavelength portion of the visible spectrum (e.g., at or near point 8) for red. If the relative radiant efficiency of these light emitting elements are similar and the fact that the eye is most sensitive to energy in the middle wavelength portion of the visible spectrum, the green light emitting element will typically have significantly higher luminance efficiency than the red or blue light emitting elements. However, this relationship may not always exist since it is plausible that the radiant efficiency of one of the light emitting elements can be significantly higher than the radiant efficiency of another light emitting element.
While one goal when designing an OLED display device is to minimize the power consumption by maximizing the efficiency of each OLED, a competing goal is to maximize the color gamut of a display device. FIG. 2 shows a CIE 1931 chromaticity diagram with the chromaticity coordinates of typical red 12, green 14 and blue 16 light emitting elements. The color gamut 18 may be defined by a triangle that connects these points within the chromaticity diagram. To improve the color gamut of the display device, the area within this triangle must be increased. To increase this color gamut, the peak wavelength of the blue light emitting element will typically be reduced, providing energy that is even shorter in wavelength and further reducing the eye's sensitivity to the radiant energy provided by the light emitting element. Similarly, to increase the color gamut, the peak wavelength of the red light emitting element must be increased, producing energy that is even longer in wavelength and further reducing the eye's sensitivity to the radiant energy provided by the light emitting element. For this reason, the goals of providing increased color gamut and reduced power consumption typically compete with one another.
Another important factor when designing a display device is that many of the colors that must be produced will be neutral or desaturated. That is, these colors will be plotted at or near the white point of the display when plotted on the CIE 1931 chromaticity diagram. For example, it is known that the predominant color on many graphic displays is white. This includes the backgrounds in many popular applications; including word processing applications, such as Microsoft Word, and operating systems, such as Microsoft Windows. Additionally, pictorial images tend to be composed of neutral or desaturated colors. This fact has also been shown in the prior art by various authors; including Yendrikhovskij, S. (2001) Computing Color Categories from Statistics of Natural Images in the Journal of Imaging Science and Technology, vol. 45, no. 5, pp. 409-417.
Therefore, to decrease the power consumption of a display device under typical use conditions, it is very important that colors near the white point of the display device consume as little power as possible. However, in a typical three-color display device, white and desaturated colors are produced by the addition of luminance from the red 12, green 14, and blue 16 light emitting elements. Since the red 12 and blue 16 light emitting elements typically have relatively low luminance efficiency, as discussed earlier, the power consumption of the display device will be near its maximum when displaying white or a desaturated color.
OLEDs formed from materials that are doped to produce different colors may also have significantly different luminance stabilities. That is, the change in luminance output that occurs over time may be significantly different for the different materials. Such different luminance stabilities can cause mismatched luminance efficiency changes to occur in the OLEDs over time, and limit the effective overall lifetime of the display device.
It is possible to utilize one or more additional light emitting elements in addition to red, green and blue elements. US2003/0011613 by Booth, Jan. 16, 2003, e.g., describes a display device with red, green, blue and cyan light emitting elements. This application discusses the fact that blue light emitting elements typically have a lower luminance efficiency than a cyan emitter. This patent application also discusses the use of a three to four color conversion matrix to convert a three-color input signal to a four-color signal. Unfortunately, utilizing a three to four color conversion using a three to four color matrix as described will result in inaccurate and desaturated primary colors. The patent application also discusses using color conversion methods such as the ones used to employ three to four or more color conversion in inkjet printing. While this body of art discusses the use of several methods to convert from three to four or more colors, there is no discussion of utilizing information such as the efficiency of a single emitter to perform the color conversion in a way that will result in lower power consumption while maintaining accurate colors.
OLED display devices having other than red, green, and blue light emitting elements have also been discussed by others. For example, U.S. Pat. No. 6,570,584 by Cok, et al., May 27, 2003 describes OLED display devices having an additional cyan, yellow, and or magenta OLEDs that are utilized to increase the color gamut of the display device. While this patent does discuss the need to convert from an input three-color input signal to a four or more color signal, it does not describe a method to utilize these OLEDs in a way to reduce the power consumption of the display device.
US2002/0191130 by Liang et al, Dec. 19, 2002 discusses a display employing pairs of complementary colors (e.g., blue, yellow, red, and green). While this patent application does not discuss a method for providing color mixing, this display device structure enables the creation of flat white fields that employ all four light emitting elements. By providing flat white fields that employ all four light emitting elements per pixel, the display provides uniform areas of near-neutral colors. However, since this method utilizes all four light emitting elements in a pixel to produce white, power consumption is not necessarily reduced.
Display systems employing three to four color conversion are also known in the art of projection displays. For example, a method proposed by Morgan et al. in U.S. Pat. No. 6,453,067 issued Sep. 17, 2002, teaches an approach to calculating the intensity of the white primary dependent on the minimum of the red, green, and blue intensities, and subsequently calculating modified red, green, and blue intensities via scaling. Additionally, Tanioka in U.S. Pat. No. 5,929,843, issued Jul. 27, 1999 provides a method that follows an algorithm analogous to the familiar CMYK approach, assigning the minimum of the R, G, and B signals to the W signal and subtracting the same from each of the R, G, and B signals. To avoid contouring artifacts that may arise due to lack of gray scale resolution, the method teaches a variable scale factor applied to the minimum signal that results in smoother colors at low luminance levels. While each of these patents discuss three to four color conversion, neither provides a method to convert from three colors to three in-gamut colors and a fourth color that is outside a triangle connecting the color coordinates of the red, green, and blue emitters when plotted in a CIE chromaticity diagram. In fact, these algorithms cannot be utilized to produce an accurate color conversion when the display device provides a fourth, gamut-expanding primary color.
A method has been proposed by Ben-Chorin in WO 02/099557 filed on Dec. 12, 2002 for providing a color conversion from a three color signal to a signal usable for wide gamut display device employing more than three primary colors. The method described, however, does not provide a means for providing this conversion in a way to reduce the power consumption or extend the lifetime of an OLED display device. The method is also inflexible in response to changing display conditions.
While Booth, US2003/0011613; Cok et al, U.S. Pat. No. 6,570,584; and Liang et al., US2002/0191130 all discuss OLED display devices having four or more primary colors and discuss the need for a three to four color conversion process, the fact that flat fields of color may be created using three or fewer of the four light emitting elements is not discussed by these authors. Further, the fact that using only three of the four light emitting elements can produce flat fields of color that do not appear uniform in luminance is also not discussed. In fact, the prior art regarding four or more primaries does not appear to discuss the dynamic adjustment of the color conversion process in response to any other display or usage parameter.
There is a need, therefore, for an improved full-color OLED display device having improved power efficiency and/or overall lifetime while maintaining accurate hues. Ideally this display device will also provide expanded color gamut and improved spatial image quality.